"start playtest game" is one of the testing options on the main menu for the game, along with the usual "start new game", "continue saved game" etc.

a playtest game starts a new game with a character with all average stats. this saves the time required to create a new player character when starting a new game (IE assigning points to stats, selecting interests, selecting a model, etc).

the only thing a playtest character gets that's special is the kit of equipment seen above. this allows you to carry things and defend yourself while testing game features. and if all else fails, there's the "N" testing hotkey to restore all your stats (including health) to 100%, and the "M" hotkey to pull up the playtest menu, which includes a "kill all animals" menu pick.